Physics and astronomy

OUR QUANTUM WORLD
Wave Particle duality of Nature

Gaurang Yodh
Physics and Astronomy
University of California Irvine

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OUTLINE

Atom and its size
Waves and Particles
Waves as particles and Particles as
Waves – Quantum View
Milestones of Quantum physics
Wave nature of Matter : de Broglie
Interference of waves
Heisenberg’s uncertainty principle
Quantum versus Classical world view
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How big are atoms ?
Democritus : Atoms as
building blocks.

Size?
Shape ?
Substance?
17000 Copper
atoms

Diameter 10­7 cm

1  nm = 10­9 meters
Atomic size determined not till the 19th  century
Atoms are very small ; about 0.5 nanometers. 
Nanotechnology deals with atomic manipulations.
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Objects

VLA Radio Tel

KECK Tel
eye
Optical microscope

Techniques of observation
/>Helps you visualize sizes

T
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Waves and Particles : What do we mean by them?
Material Objects:
Ball, Car, person, or point like objects called particles. 
They can be located at a space point at a given time.
They can be at rest, moving or accelerating.
Falling Ball

Ground level

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Waves and Particles: What do we mean by them ?
Common types of waves: 
Ripples, surf, ocean waves, sound waves, radio waves.
Need to see crests and troughs to define them.
Waves are oscillations in space and time.
Direction of travel, velocity
Up­down
oscillations

Wavelength ,frequency, velocity  and oscillation size defines waves 
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Particles and Waves: Basic difference in behaviour
When particles collide they cannot pass through each other ! 
They can bounce or they can shatter
Before collision 

After collision

Another after
collision state
shatter 

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Collision of truck with ladder on top with a
Car at rest ! Note the ladder continue its
Motion forward ….. Also the small care front
End gets smashed.

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Head on collision of a car and truck

Collision is inelastic – the small car is dragged along
By the truck……

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Waves and Particles Basic difference:
Waves can pass through each other !
As they pass through each other they can enhance or cancel
each other
Later they regain their original form !

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Waves and Particles: 

Wavelength

Frequency

Spread in space and time
Waves

Can be superposed – show 
interference effects

Pass through each other

Localized in space and time
Particles
Cannot pass through each other ­
they bounce or shatter.
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OUR QUANTUM WORLD
In the 20th century, study of atomic systems required a 
fundamental revision of these classical ideas about 
physical objects.
1. Light waves exhibited particle like properties – 
  phenomena called photo­electric effect in which light impinging
  on certain metals cause instanteous emission of electrons
  in a billiard ball like impact.  
 – the basis of automatic door openers in grocery stores 
2. Electrons (particles) exhibit wave like properties – they
 can pass through each other ! Phenomenon of  
    electron interference –  basis of electron microscopes

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OUR QUANTUM WORLD

.
This quantum picture of the world is at odds 
with our common sense view of physical objects.
We cannot uniquely define what is a particle and
what is a wave !!
Neils Bohr and Werner Heisenberg were the architects of this 
quantum world view, along with Planck, Einstein, de Broglie,
Schrodinger, Pauli and Dirac. 

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TRUE UNDERSTANDING OF NATURE REQUIRED
THAT PHYSICAL OBJECTS, WHATEVER THEY ARE,
ARE NEITHER EXCLUSIVELY PARTICLES OR WAVES
No experiment can ever measure both aspects at the same 
time, so we never see a mixture of particle and wave.
WHEN ONE OBSERVES A PHYSICAL PHENOMENON
INVOLVING A PHYSICAL OBJECT,  THE BEHAVIOUR
YOU WILL OBSERVE – WHETHER PARTICLE LIKE OR
WAVE LIKE – DEPENDS ON YOUR METHOD OF 
OBSERVATION. 
THE OBJECT IS DESCRIBED BY  MATHEMATICAL 
FUNCT IONS WHICH ARE MEASURES OF PROBABILITY . 
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MILESTONES OF QUANTUM PHYSICS:
J.J.Thomson  Established electron as a fundamental
particle of nature. He measured its charge to mass
ratio using a Crooke's tube.
Electric current = flow of electrons

Crooke's tube:
Evacuated tube
Visualization of 
electron beam.

Animation of electrons moving
and being deflected by an electric
or magnetic field. 

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Marie Curie and Radioactivity - 1898

Discovered that certain elements ‘ spontaneously ‘
emit radiations and change into different elements.

Only woman scientist to receive two Nobel Prizes:
One in chemistry and the other in physics.

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The Quantum of Light or the Photon
Particle nature of light was proposed by 
Einstein in 1905 to explain the photo­electric
effect. Photo­electric effect – automatic door
openers in grocery stores. Particles of light
are called light quanta or photons.
Energy of a Photon = h (frequency of light)
 h is a fundamental constant of nature and it is
 very small in size.
Packet of energy in photon is so small that we are not aware
of the rain of photons of light impinging on our eyes – just as
 you cannot feel the impact of individual air molecules, you 
 only feel a breeze.
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Rutherford and his Nuclear Atom: 1898 ­1911
R
r
Ernest Rutherford used alpha rays to discover the 
nucleus of the atom. The nucleus was positvely
charged and contained almost all of the mass of
the atom.  Most of the atom was empty space.  
Atomic size

Electron cloud

Classical physics required
that this atom is unstable
electrons would fall into
the nucleus in 10­7 sec!
Nuclear size

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Planck and quantization of atomic “ vibrations “
Before Einstein, Planck postulated from study
of radiation from hot bodies that the radiating
atoms can only radiate energy in discrete amounts
– or that atoms exist only in discrete states, called
Quantum states.
This was the birth of quantum physics in 1900

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THE BOHR ATOM: 

Bohr proposed a revolutionary model:

An atom with discrete (Quantum) states
– an ad hoc model

Bohr model explained how atoms emit light
quanta and their stability. He combined the
postulates of Planck and Einstein to build
characteristic energy states that atoms should
possess. Model gave excellent agreement with
experiment on atomic spectra.(1913)
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Bohr atom

Bohr’s atom model achieved three important results:
1. Atoms are stable
2. Different atoms of the same element are identical
3. Atoms regenerate if they are taken apart and
then allowed to reform.

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THE BOHR ATOM: 

Understanding the origin of Bohr's model required an

essential bold step – enter  Louis de Broglie.

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Wave nature of material bodies:

If light, which classically is a wave,
can have particle nature
As shown by Planck and Einstein,
Can material particles exhibit wave nature ?
Prince Louis de Broglie while doing
his Ph.D. research said particles
should have wave like properties.
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Wave Nature of Matter
Louis de Broglie in 1923 proposed that
matter particles should exhibit wave 
properties just as light waves exhibited
particle properties. These waves have 
very small wavelengths in most situations
so that their presence was difficult to observe
These waves were observed a few years later by Davisson and
G.P. Thomson with high energy electrons. These electrons show

the same pattern when scattered from crystals as X­rays of similar
wave lengths.
Electron microscope
picture of a fly

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A SUMMARY OF DUAL ITY OF NATURE
Wave particle duality of physical objects
LIGHT
Wave nature ­EM wave

Particle nature ­photons

Optical microscope

Convert light to electric current
Photo­electric effect

Interference
PARTICLES
Wave nature
Matter waves ­electron
microscope

Particle nature
Electric current

photon­electron collisions

Discrete (Quantum) states of confined
systems, such as atoms.
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